U.S. warplanes conducted airstrikes on forces believed to be loyal to Syrian President Bashar Assad in southern Syria on Thursday as a protection measure for U.S. forces based in the region, U.S. officials said.

The airstrikes, in the southern town of Tanf, mark the first time that the U.S. military deliberately hit the regime out of a perceived threat for American troops, which have steadily increased in numbers in recent months throughout Syria.

U.S. commanders became concerned when they saw tanks, bulldozers and other heavy equipment advance into a "de-confliction" zone without authorization near a base where U.S. forces were located, according to the officials, who spoke about the incident after requesting anonymity because they were not authorized to discuss the matter publicly.

It was not clear if the advancing forces were Syrian army troops or other pro-government allies, but they were flying Syrian flags and began constructing berms and fighting positions, officials said. It's been publicly known for nearly a year that U.S. special operations forces operate out of the base, near where the borders of Jordan, Iraq and Syria meet, training Syrian rebels in their fight against Islamic State militants. Pentagon officials have said in the past that the U.S. military would defend its forces if they were threatened inside Syria by Assad's government.

The U.S. military does not speak with the Syrian government. So U.S. military commanders, at an air command center in Qatar, called their Russian counterparts on a special hotline set up to ensure the two countries' pilots do not mistakenly run into -- or fire upon -- one another in the skies above Syria. After warning the Russians of the impending strikes, the United States flew near the Syrian ground forces and fired a warning shot as a "show of force," officials said. After the ground forces continued their activity, the U.S. carried out the airstrike, the officials said.

The strike is the second time this year the U.S. military has targeted Syrian forces. The U.S. military attacked a Syria-government airfield with 59 Tomahawk cruise missiles on April 6 in response to a chemical attack that killed dozens of civilians, including children.